src: use subarray() in Buffer#slice() for speedup

Use the built-in Typed Array method subarray() to improve performance of
Buffer#slice().

Benchmark improvements:

    benchmark/buffer-slice: 40%
    benchmark/buffer-creation (pool): 25%

Additional tests also added.

PR-URL: https://github.com/nodejs/node/pull/2777


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>